NEODyS-2

Near Earth Objects - Dynamic Site
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Data used in this tool are about unconfirmed objects and all information should therefore be treated as potentially unreliable.

More formally, the impact probabilities computed by the software behind this visualization tool are based on observation error statistics, assigned by means of an astrometric observations error model based upon Gaussian (normal) distributions. Because the number of individual observations for each object is very small, the law of large numbers does not apply. Thus the actual errors of the observations included in a single tracklet, normally between 3 and 5, may not be a representative sample of the corresponding random variable, which is normally distributed.

In simple words, the probability for a single tracklet to have large errors is small, but not as small as detecting an imminent impactor (less than 1 in a million tracklets). Thus some apparent detections of imminent impactors can be spurious, and there is no way to avoid this, short of abandoning the statistical error model and resorting to a careful human inspection of the image to reveal possible causes of degradation of the data.


General description

NEOScan is a system dedicated to the scan of the Minor Planet Center NEO Confirmation Page (NEOCP). The goal is to identify asteroids as NEOs, MBOs or distant objects to confirm or remove from the NEOCP, and to give early warning of imminent impactors, to trigger immediately follow-up observations.

The service NEOScan is based on the algorithm presented in Spoto et al. (2018), which we are going to summarize.


Impact flag

We assign to each NEOCP object an integer flag related to the computation of its impact probability. It is called impact flag, and it depends on the impact probability value and on the arc curvature, as shown in the table below. We say that an arc has significant curvature if χ2 > 10, where χ is the chi-value of the geodesic curvature and the acceleration. The impact flag can take the integer values from 0 to 4: a 0 value indicates a negligible chance of collision with the Earth, whereas the maximum value 4 express an elevated impact risk. It is conceived as a simple and direct communication tool to assess the importance of collision predictions, and to give the priority for the follow-up activities.

Impact flag Condition Risk
0 IP ≤ 10-6 Negligible
1 10-6 < IP ≤ 10-3 Very small
2 10-3 < IP ≤ 10-2 Small
3 IP > 10-2 and no significant curvature Moderate
4 IP > 10-2 and significant curvature Elevated


Score computation

When we perform the sampling of the Admissible Region, we also compute a score for the object. The score gives us a first insight into the nature of the object, even though the asteroid were not a potential impactor. The score is a rigorous probability of the object to belong to different classes (NEO, MBO, DO, and SO).

The systematic ranging allows one to express the orbital elements as a function of the range and the range-rate. Therefore, each orbit on the Manifold Of Variations can be matched to one of the four classes. Their definitions are listed below.

where a is the semi-major axis (in au), and e is the eccentricity. In these way, each of these classes corresponds to a subset of the range/range-rate space, and thus its probability can be computed.